A standard hammer or percussion device such as described in U.S. Pat. No. 4,508,017 and in commonly assigned copending patent application No. 886,546 of J. C. Bartomeuf has a housing forming a cylinder in which is provided a piston that is longitudinally reciprocal and that defines in the cylinder an upper or rear compartment and a lower or front compartment separated longitudinally therefrom by the piston. This piston moves forward, that is longitudinally toward the front compartment, to strike the rear end of a tool used to dig, chip or the like, and backward to return to a starting position spaced longitudinally behind the tool. The force for forward motion, which is normally downward, comes from the pressure differential between the front and rear compartments while the force for backward or return motion comes in part from this differential and in part from the piston rebounding from the tool.
Efficiency, chiefly penetration, is maximized when working on a hard workpiece by using blows of a relatively low frequency but of considerable energy while with a soft workpiece light blows of high frequency are used. The energy imparted to the tool by the piston at the forward end of its stroke is directly related to the piston speed at this instant. This speed can be increased by increasing the pressure differential moving the piston to increase its acceleration or by increasing the length of the piston's stroke to give it more time to get up to the speed and this speed can be decreased by oppositely varying these parameters. The frequency is controlled mainly by a reversible distributing valve connected on the one side to high- and low-pressure sides of a source of fluid and on the other side of the front and rear compartments longitudinally flanking the piston as well in stroke-adjustable models to ports defining the front end of the rear compartment or rear end of the front compartment.
Thus it is not impossible for such a device to have independent adjustment knobs, levers, or the like for force and frequency. Readjustment is necessary when the workpiece changes so that it is necessary to trade off time wasted adjusting the tool on one hand against effectiveness of the tool on the other.